Sweat sensing technologies have enormous potential for applications ranging from athletics, to neonatology, to pharmacological monitoring, to personal digital health, to name a few applications. Sweat contains many of the same biomarkers, chemicals, or solutes that are carried in blood and can provide significant information enabling one to diagnose illness, health status, exposure to toxins, performance, and other physiological attributes even in advance of any physical sign. Furthermore, sweat itself, the action of sweating, and other parameters, attributes, solutes, or features on, near, or beneath the skin can be measured to further reveal physiological information.
Sweat biosensing applications have largely remained relegated to infant chloride assays for Cystic Fibrosis or illicit drug monitoring patches. This is because the majority of medical literature on sweat biomarkers relies on a slow and inconvenient process of sweat stimulation, collection of a sample, transport of the sample to a lab, and then analysis of the sample by a bench-top machine and a trained expert. This process is so labor intensive, complicated, and costly that a blood draw is usually a superior testing modality, since it is the gold standard for most high performance biomarker sensing. Hence, sweat sensing has yet to realize its full biosensing potential, especially for continuous or repeated biosensing. Furthermore, attempts to use sweat sensing to measure “holy grails” like glucose have thus far failed to produce viable commercial products, reducing the perceived capability and opportunity space for sweat sensing.
Among the physiological fluids used for bio monitoring (e.g. blood, urine, saliva, tears), sweat has arguably the least predictable sampling rate in the absence of technology. However, with proper application of technology, sweat can be made to outperform other non-invasive or less invasive biofluids in predictable sampling. For example, it is difficult to control saliva or tear rate without negative consequences for the user (e.g., dry eyes, tears, dry mouth, or excessive saliva while talking). Urine is also a difficult fluid for physiological monitoring, because it is inconvenient to take multiple urine samples, it is not always possible to take a urine sample when needed, and control of biomarker dilution in urine imposes further significant inconveniences on the user or test subject.
By contrast, as disclosed in herein, sweat may be stimulated and sampled when needed, and sweat rates may be controlled for particular applications. For instance, some biomarkers diffuse into sweat at known rates, and these rates will correspond to a particular sweat rate that allows the biomarker's sweat concentration to optimally correlate to its concentration in blood. (e.g., too high of a sweat rate will dilute a biomarker concentration as the biomarker may not have time to equilibrate by diffusion into sweat). An excellent summary is provided by Sonner, et al. in the 2015 article titled “The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications”, Biomicrofluidics 9, 031301, herein included by reference.
Many of the drawbacks and limitations stated above can be resolved by creating novel and advanced interplays of chemicals, materials, sensors, electronics, microfluidics, algorithms, computing, software, systems, and other features or designs, in a manner that affordably, effectively, conveniently, intelligently, or reliably brings sweat sensing and stimulating technology into intimate proximity with sweat as it is generated. With the improvements embodied in the current invention, sweat sensing can become a compelling new paradigm as a biosensing platform.